The absorptive and secretory processes which characterize the large intestine are well understood, but must represent fundamentally the movement of salts and water across a single layer of epithelial cells. Evidence has accumulated which suggests that transmural movements in epithelia may proceed via two parallel paths, one through the cells and a second circumventing the cells via paracellular shunt pathways. In addition, there is ample evidence that in order for such tissues to exhibit active transport of Na, or other ions, the cell membranes which form the functional boundaries of the epithelium must have fundamentally different properties. The proposed studies will be directed primarily toward obtaining a quantitative description of the roles of cellular and shunt pathways in transmural ion movement across the amphibian and reptilian colon and evaluating the properties of the cell membranes which make up the mucosal and serosal borders of the cell layer. Specifically, we propose to undertake a detailed study of the movements of Na, Cl, and K across the isolated colonic mucosa of toads and turtles and attempt to more clearly define the mode of action of hormones such as aldosterone, vasopressin, and angiotensin II which modify transport processes in these tissues.